My advice would be to buy the biggest one that will fit in the space you have available, you'll regret it if you don't :) - and yes, you can put a GH on concrete slabs. Be sure to tell your supplier you need a base that can be properly anchored.
I envy you being able to order just what you need, sigh :( I had to have my 'jobber' construct one for me in the tiny space I have. Cost a fortune too, but I wouldn't be without it for bringing on seedlings - and my friend has a larger one 8 x 6 where I can grow lots of summer veggies. ;)
Tricia